If read() syscall requests unexpected number of bytes from "dimm" binary
attribute file, return EINVAL instead of EPERM.

At the same time pin down sysfs file size to the fixed
sizeof(struct netxen_dimm_cfg), which allows to exploit some missing
sanity checks from kernfs (file boundary checks vs offset etc.)
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>